1. Field of the Invention
The present invention relates to an image forming apparatus provided with a charging means which evenly charges an image bearing member using electroconductive, magnetic particles, suitably used for electrophotographic copiers and their printers, in particular.
2. Related Background Art
(1) Charging Means
Charging means for even charging treatment (including elimination treatment) of an image bearing member, e.g., electrophotographic photosensitive member, electrostatic recording dielectric or the like, to secure a given potential of given polarity on the carrier surface in image forming apparatuses fall into two general categories, non-contact and contact types.
a) Non-contact Charging Means
A corona charging device (or discharging device) is a non-contact charging means facing an image bearing member (hereinafter referred to as photosensitive member) in a non-contact manner to expose the photosensitive member surface to a corona shower discharged when a high voltage is applied and thereby to charge the surface at a given potential of given polarity with a discharge-generated product.
b) Contact Charging Means
A contact charging means brings an electroconductive, charging members, e.g., roller (charging roller), fur brush, magnetic brush, blade or the like, to a photosensitive member, and applies a given charging bias to the photosensitive member surface to charge the surface at a given potential of given polarity. It has advantageous for the low ozone production and low power requirement, among others, over a corona charging device.
A charging bias may be applied to a contact charging means either by a DC bias method in which only a DC bias is applied and AC bias method in which a DC bias overlapped with an AC bias is applied.
A charging mechanism (charging mechanism or principle) for contact charging means is a mixture of corona charging and contact injection charging systems, and its characteristics are mainly determined by those of the predominant one.
A corona charging system produces a discharge phenomenon, e.g., corona discharge, in a fine gap between the contact charging means and photosensitive member, to charge the photosensitive member surface with a discharge-generated product. A corona charging system produces trace quantities of ozone, although to a much lower extent than a corona charging device.
A contact injection charging system directly injects charges from a contact charging member into a photosensitive member to charge the photosensitive member surface. This procedure is sometimes referred to as direct charging or injection charging. Japanese Patent Application Laid-Open No. 6-3921 proposes a method for contact injection charging in which charges are injected into a charge-holding member, e.g., trap level on a photosensitive member surface or electroconductive particles in a charge injecting layer, by contact charging member, e.g., charging roller, charging brush or charging magnetic brush.
An organic photosensitive member capable of charging by contact injection, for example, should be coated with a charge injecting layer dispersed with fine, electroconductive particles as a charge-holding member. On the other hand, an inorganic photosensitive member, beginning with that of amorphous silicon, needs no charge injecting layer anew, because it has a number of trap levels resulting from crystal defects on the surface, which can hold the injected charges for charge injection.
Contact injection charging is not based on a discharge phenomenon, and only needs a photosensitive member surface potential as charging voltage. Therefore, it is an ozone-less, low-power charging method. Moreover, it can theoretically increase a surface potential of the charged member to a voltage applied thereto, and makes the member resistant to changes in ambient conditions, e.g., moisture.
On the other hand, contact injection charging injects charges into a photosensitive member only through a surface area at which it is in contact with a charging member, by which is meant that its charging capacity is determined by ratio of contact between a charging member and photosensitive member. When there is a large uncharged area left at an insufficient contact ratio, charging may be terminated before surface potential on a photosensitive member reaches a voltage applied to a charging device.
The effective charging methods for securing a high contact ratio evenly over an entire area to be charged include bringing a magnetic brush composed of magnetically restricted, electroconductive, magnetic particles in contact with a photosensitive member, and bringing an elastic roller of electroconductive sponge or the like with fine, electroconductive particles deposited thereon in contact with a photosensitive member via the fine particles.
The former method generally brings magnetic particles in contact with a photosensitive member, where a charging bias is applied to an electroconductive, rotational sleeve containing a multipolar magnet roller, placed in the vicinity of the photosensitive member to hold the magnetic particles thereon by its magnetic force, with quantity of the particles being controlled and uniformized by a doctor blade as a magnetic particle restricting member.
The latter method brings fine, electroconductive, magnetic particles, deposited on an electroconductive, sponge roller with fine pores, in contact with a photosensitive member, where a charging bias is applied to the roller. The fine particles expand the electrical contact area between the roller and photosensitive member, and, at the same time, work to reduce friction between them and further increase contact probability between them, for which the sponge roller is driven to rotate based on difference in peripheral velocity between them.
c) Plural Magnetic Brush Charging Means
Charge injection in contact injection charging is based on a charging phenomenon in a condenser with an electroconductive board of a photosensitive member and contact area of a charging member as electrodes. Therefore, a certain extent of charging time is theoretically needed to secure a desired potential. Increasing process speed decreases time for charges to pass through a contact area, when it is set constant, leading to shortened charging time, with the result that a desired potential may not be secured. An inorganic photosensitive member, e.g., that of amorphous silicon, has a higher dielectric constant than an organic photosensitive member, and needs more charges and hence a longer charging time. A longer charging time is also needed, when a toner or additive for a toner cannot be removed by a cleaning device and is deposited on electroconductive, magnetic particles to increase their resistance.
Japanese Patent Application Laid-Open No. 8-44153 proposes an image forming apparatus provided with plural magnetic brush charging means, in order to solve these problems. This apparatus charges a photosensitive member by an upstream charging means located in the upstream of the photosensitive member rotation direction and additionally by a downstream charging means to secure a desired potential on the photosensitive member, because the upstream means alone cannot sufficiently secure the potential. In other words, it charges a photosensitive member 2 or more times to control charging-related problems, e.g., uneven contact or resistance of a charging member even at a high process speed, because of extended charging time. Moreover, a desired potential on the photosensitive member can be secured at a reduced charging load for an individual charging device, because potential fluctuations can be controlled even when the charging member has an increased resistance caused by its contamination or changed ambient conditions. As a result, this brings another advantage of easily expanding service life of the apparatus.
However, a structure with plural magnetic brush charging means involves the following problems.
The photosensitive member has an insufficient potential relative to voltage which the upstream magnetic brush charging means applies thereto while it is passing over the photosensitive member, with the result that contrast between the charging means and photosensitive member surface is expanded. This increases intensity of an electrical field produced by the charging means for the photosensitive member, to accelerate deposition of the magnetic particles on the photosensitive member. The magnetic particles left on the photosensitive member by the upstream magnetic brush charging means causes uneven potential between the surface portion on which the particles are deposited and particle-free portion while the downstream magnetic brush charging means is charging the photosensitive member. It is therefore necessary to control, as far as possible, deposition of the magnetic particles of the upstream magnetic brush charging means on the photosensitive member, for which the charging means should have a sufficient magnetic force to hold the particles.
On the other hand, the downstream magnetic brush charging means involves its own problems. Potential contrast between the photosensitive member and downstream means contracts while the latter is passing over the former, because of increased potential of the former. This prevents deposition of the magnetic particles on the photosensitive member. However, resistance may not be evenly distributed on the photosensitive member surface, when the magnetic brushes are not even in the gap between the photosensitive member and downstream means, to cause uneven potential distribution on the photosensitive member surface after the means leaves. It is therefore necessary to reduce uneven magnetic brushes in the gap between the downstream means and photosensitive member.